Move many functions from _testcapimodule.c into more specific files
in Modules/_testcapi/.
In moved code:
* Replace get_testerror() with PyExc_AssertionError.
* Replace raiseTestError() with
PyErr_Format(PyExc_AssertionError, ...).
The new `PyList_GetItemRef` is similar to `PyList_GetItem`, but returns
a strong reference instead of a borrowed reference. Additionally, if the
passed "list" object is not a list, the function sets a `TypeError`
instead of calling `PyErr_BadInternalCall()`.
* Split list_extend() into two sub-functions: list_extend_fast() and
list_extend_iter().
* list_inplace_concat() no longer has to call Py_DECREF() on the
list_extend() result, since list_extend() now returns an int.
Victor Stinner